This invention relates to an alkaline storage battery such as a nickel-hydrogen battery, a nickel-cadmium battery, and a nickel-zinc battery, and in particular this invention relates to an alkaline storage battery containing a separator having a unique structure.
In recent years portable electronic equipment represented by such devices as portable word processors, portable telephones, portable personal computers, and video cameras have had an increasing tendency to become light-weight and miniaturized. To advance this miniaturization of electronic equipment, a demand for a new level of battery miniaturization and performance has taken place.
Particularly, a strong demand has developed for a high battery capacity in typical alkaline storage batteries, such as nickel-hydrogen and nickel-cadmium batteries. A key issue revolves around increasing the energy density of the positive and negative electrode materials and at the same time reducing the separator thickness.
One simple method of reducing separator thickness is to decrease the weight of the separator per unit surface area. However, a simple reduction of separator weight per unit surface area with no other adjustments leads to a drastic reduction in the mechanical strength of the separator. This results in breaks in the separator during battery assembly when the positive and negative electrodes are wound with the separator between them. Even supposing that successful electrode winding is possible, micro-cracks developing in the positive and negative electrode plates lead to separator damage and numerous short circuits.
Japanese Non-examined Patent Publication No.3-257755 issued Nov. 18, 1991 recites one method of obtaining a separator with excellent ability to retain electrolytes. This disclosure describes a treatment with a high pressure water stream to break apart and interconnect composite segmented fibers of a non-woven fabric separator. The non-woven fabric separator is made up of two types of composite segmented fibers using resins with superior chemical resistance and resins having excellent hydrophilic properties.
This type of high pressure water treatment to break apart and interconnect composite segmented fibers of a non-woven fabric separator not only provides a separator with excellent ability to retain electrolytes, but also promises to improve the separator's mechanical strength. Specifically, by increasing the interconnections of fibers that make up the non-woven fabric in three-dimensions, bonding between fibers is expected to becomes stronger thereby improving mechanical strength.
However, when a non-woven fabric separator with increased fiber interconnection due to high pressure water treatment is used in an alkaline storage battery, the battery develops the problem of operating under abnormally high internal pressure. This is caused by decreased separator permeability due to strong retention of electrolyte within the fiber gaps of the nonwoven fabric with increased interconnection between fibers. When separator permeability is decreased, gas generated at the positive electrode cannot smoothly pass through the separator. Gas which does not quickly pass through the separator cannot be absorbed by the negative electrode and the internal pressure of the alkaline storage battery increases.
Another separator using composite segmented fibers is described in Japanese Non-examined Patent Publication No.5-182654 issued Jul. 23, 1993. This disclosure describes a three layered separator in which water stream interconnected non-woven fabric is bonded to both sides of melt-blow non-woven fabric to improve electrolyte retention without sacrificing mechanical strength.
However, it is difficult to make this three layered separator both strong and readily gas permeable. Insufficient strength results from the weakness of the core melt-blow non-woven fabric. In particular, since the separator structure is a laminate of three layers of non-woven fabric bonded together, reduced strength results because the core melt-blow non-woven fabric cannot be made sufficiently thick. Another reason for reduced separator strength is difficulty in achieving an ideal bond between the melt-blow and water stream interconnected non-woven fabrics. If the amount of bonding adhesive used to laminate the non-woven fabrics is increased, the bonding adhesive will close off gaps between fibers and thereby reduce separator permeability. Further, if the non-woven fabrics are sufficiently heat-welded together, the laminate becomes compressed, thereby reducing inter-fiber gaps and gas permeability. Consequently, this separator structure results in a reduction in gas permeability and an increase in internal battery pressure when the melt-blow and water stream treated non-woven fabrics are strongly bonded together. In addition, this separator structure results in a reduced separator strength when the gas permeability is improved. In other words, the properties of separator strength and gas permeability are in mutual opposition and improvement of both is extremely difficult. Finally, since the thin separator described in this disclosure is fabricated by bonding water stream interconnected non-woven fabric to melt-blow non-woven fabric, manufacture is extremely complex and it is difficult to achieve low cost, high quantity production low cost.
The present invention was developed to solve the above mentioned problems. It is thus a primary object of the present invention to provide a high capacity, long lifetime alkaline storage battery having a separator which can be manufactured inexpensively in quantity with both improved strength and permeability and which has reduced thickness without sacrificing electrolyte retention.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.